In a letter sent to the Senate and the House Foreign Affairs committees July 7, Mattis expressed concerns about the defense budget, known as the National Defense Authorization Act.

"At this time, I oppose the removal of Turkey from the F-35 program," he said. "Turkey is a NATO ally and has been a program participants since 2002, investing $1.25 billion, contributing to the supply chain, and plans to procure 100 F-35As."

Removing Turkey could cause disruption in a supply chain for the U.S. military and its partners while increasing other program costs, Mattis said.

"If the Turkish supply chain was disrupted today, it would result in an aircraft production break, delaying delivery of 50-75 F-35s, and would take approximately 18-24 Months to re-source parts and recover," he added.

Turkey has been in the F-35 program since 1999. The Turkish defense industry has taken an active role in the production of parts for the F-35, including Alp Aviation, AYESAS, Kale Aviation, Kale Pratt & Whitney and Turkish Aerospace Industries.

The Senate, however, overwhelmingly approved the annual National Defense Authorization Act, which includes an amendment prohibiting sales to Turkey of the jets, citing the purchase of the S-400 air defense system from Russia and the detainment of U.S. citizens as reasons.

The bill must now be reconciled with one already passed by the House of Representatives in May and a compromise measure must then be passed by both chambers and signed by President Donald Trump.

Going along with the congressional concerns about an "authoritarian drift in Turkey" and the detainment of Americans, including Pastor Andrew Bronson, Mattis said the Donald Trump administration is pressing Ankara on these issues as well as the potential acquisition of the S-400 air defense system.

Brunson was arrested in 2016 on charges of committing crimes on behalf of the PKK terror group and the Fetullah Terrorist Organization (FETO), the group behind the defeated coup attempt in July 2016.

Turkish Foreign Minister Mevlut Cavusoglu previously criticized Washington for trying to force Turkey not to purchase the S-400 defense system from Russia, claiming that Turkey had been trying to purchase the Patriot air defense system from the U.S., but the missiles have never been sold to Turkey as they would be valuable in the fight against the PKK and YPG and the threat from Syria.